fix(test): prevent flaky deadlock in throttle backlog test (#5474)

The `held` channel in `runTwoRequests` was unbuffered, creating a race
condition with the `select/default` send in the handler. Under CI load
(slow runner, -race, -shuffle=on), the handler goroutine could reach
the select before the test goroutine blocked on `<-held`, causing the
send to silently fall through to `default` and deadlocking both
goroutines permanently.

Buffer the channel (capacity 1) so the send always succeeds regardless
of goroutine scheduling order.

server/throttle_backlog_test.go

@@ -209,7 +209,7 @@ var _ = Describe("ThrottleBacklog", func() {
 // runTwoRequests sends two concurrent requests through a throttled router. The
 // first request holds the token until the second has been dispatched.
 func runTwoRequests(m func(http.Handler) http.Handler) (firstStatus, secondStatus int) {
-	held := make(chan struct{})
+	held := make(chan struct{}, 1)
 	release := make(chan struct{})
 	r := chi.NewRouter()
 	r.Use(m)